1. Field of the Invention
The present invention relates to a projection optical system and an image projection apparatus that project images, and more particularly, to a projection optical system and an image projection apparatus that improve resolution by changing the image position on the projection surface.
2. Description of the Related Art
An image projection apparatus that projects images onto a screen comprises a light valve displaying images and a projection optical system directing light from the light valve representative of an image to a screen to form the image on the screen. The light valve has a multiplicity of pixels arranged with a predetermined pitch, and displays one point of an image for each pixel. Consequently, the resolution of the image is determined by the number of pixels, and when the size of the light valve is fixed, the resolution is determined by the arrangement pitch of the pixels.
While there are various kinds of light valves based on different display principles such as LCDs (liquid crystal displays) and DMDs (digital micro-mirror devices), there is a kind where a part displaying no image is present between pixels. For example, LCDs are provided with a belt-shaped light intercepting portion called a black matrix for the disposition of a driving circuit and the prevention of light mixture. The arrangement pitch of the pixels depends on the pixel size, and the resolution improves as the pixel size decreases. However, in the light valve having a structure such that a part displaying no image is present between pixels as mentioned above, there is a limit to the improvement of the resolution.
In order that the resolution of the provided image is higher than that of the image displayed by the light valve, Japanese Laid-Open Patent Application No. H03-198037 proposes to periodically shift the projection on the screen on the order of the pixel pitch (⅓ to one pixel) and change the displayed image according to the shift. By doing this, for example, a point on the screen corresponding to the part between pixels of the light valve at a certain point of time can be made to correspond to the center of the pixels at the next point of time, so that the resolution of the projected image is improved.
As a method of shifting the projected image, Japanese Laid-Open Patent Application No. H03-198037 discloses to tilt the mirror directing illumination light to the light valve, move the light source emitting the illumination light, move the light valve and tilt or move a lens unit in the projection optical system.
When the projected image is shifted by moving a lens unit in the projection optical system, it is necessary to consider that the quality of the projected image changes by the movement of the lens unit. Moreover, it is necessary to move the lens unit at high speed so that the viewer does not notice the image shift on the screen. Further, it is necessary to control the position of the lens unit with accuracy. However, this patent application makes no reference to these.
Normally, in a case where a lens unit included in the projection optical system is decentered (moved in a direction vertical to the optical axis), when the movement amount is δ, the point on the light valve corresponding to the same point on the screen is shifted by approximately 0.5δ to 1.5δ. That is, in a case where a light ray is made incident on the projection optical system from the opposite direction, when the lens unit is decentered by δ, the position of the light ray on the light valve is changed by approximately 0.5δto 1.5δ. Hereinafter, a ratio of the shift amount of the position on the light valve corresponding to a point on the screen to the movement (decentering) amount of the lens unit will be referred to as decentering sensitivity.
In recent standard light valves, since the pixel arrangement pitch is 10 μm to 20 μm, in order to cause a pixel shift of approximately the arrangement pitch in the projected image, the lens unit is moved by approximately 6 μm to 40 μm. The lens unit in the projection optical system has, in the case of a high performance lens unit, an effective diameter Φ of not less than approximately 30 mm although it depends on the size of the light valve and the lens back focal length. To mechanically drive such a large lens unit with a high accuracy of 6 to 40 μm, extremely highly accurate processing and adjustment are required. Under the present circumstances, an actuator suitable for such a high accuracy is absent. A position sensor detecting the position of the lens unit to change the image according to the position of the lens unit which sensor is suitable for such accuracy is also absent.
The lens units included in the projection optical system are fixed to the lens frame with extremely high accuracy, and their performances deteriorate when a tilt error or the like occurs. When any of the lens units is capable of being decentered, a tilt error or the like readily occurs on the lens unit, which results in degradation in the performance of the projection optical system. In particular, when a lens unit having strong optical power which lens unit is essential to projection is decentered, performance degradation cannot be avoided, so that the image quality is degraded although the resolution is improved.